1. Field of the Invention
The present invention concerns an electronic protection device for thermal protection of a load such as a rotating electrical machine or a distribution circuit by cutting off the multiphase supply of power to the load in the event of an imbalance between phase currents or a current overload on at least one phase, the in device finding a specific application thermal protection relays and circuit-breakers.
2. Description of the Prior Art
Existing thermal protection relays protect rotating electrical machines or distribution circuits from overloads by monitoring the current absorbed on each phase or detecting phase imbalance or absence of a phase. A thermal protection relay includes a bimetal strip for each phase which is heated by the phase current. If an overload occurs on any phase the bimetal strip of the overloaded phase bends and mechanically operates a tripping mechanism which opens contacts to cut off the supply of power to the rotating electrical machine or the distribution circuit. Tripping in response to a phase imbalance or absence of a phase is effected by a differential device.
These thermal protection relays are difficult to use if the current is greater than around 100 amperes. Heating of the bimetal strip and therefore the tripping current are difficult to control because of spurious heating effects.
Motors are usually protected by electronic protection relays such as those described in French patent 2 586 489. These relays include a current sensor for each phase producing an image of the phase current. An electronic processor circuit receives and processes the phase current images in order to output a tripping instruction if necessary.
An electronic protection relay described in French patent 2 641 410 is provided with a thermal sensor in the form of resistive heating elements through which the phase currents flow and which are thermally coupled to a temperature sensor by means of a thermal coupling body. The thermal sensor provides a simple way of memorizing the thermal state of the load protected by the relay.
An object of the invention is to provide an electronic protection device which is capable of detecting current overloads and/or phase current imbalance at high currents and using a thermal sensor employing silkscreen-printed resistive elements. This device is insensitive to harmonics of the currents.